A conventional system for modifying data stored in non-erasable read-only memory devices (such as Read Only Memory, hereafter described as ROM) is disclosed in Japanese Patent Kokai Publication JP-A No. Hei 7-129396 (1995). However, the publication does not describe a technology for patching compressed data that is stored in memory. Therefore, when compressed data stored in ROM must be modified to increase functionality or to correct errors, ROM must be replaced regardless of the amount of correction.
Another conventional system for patching a micro-program or a program, at a low cost, is disclosed in Japanese Patent Kokai Publication JP-A No. Hei 8-44551 (1996). The technology requires two types of data, that is, compressed data that is to be patched and patch data that is used to patch the compressed data, to reside in the same device. Therefore, when the memory device is a non-erasable read only device such as ROM, ROM must be replaced even if only patch data must be replaced